1. Field of the Invention
The present invention relates to an air filter material to be used in an air cleaner for various types of internal combustion engines that run on gasoline, ethanol, light oil, heavy oil, or other fuels, including four-stroke or two-stroke reciprocating engines, rotary engines, and gas turbine engines. In particular, the present invention relates to an air filter material intended for engines of automotive vehicles (such as a motorcycle and a four-wheel car), which has excellent capabilities to collect carbon soot as well as to collect sand and other dusts (see JIS D-1612) and is further capable of suppressing a dust penetration phenomenon which tends to occur under particular conditions.
2. Description of the Related Art
If dust-containing air is introduced into cylinders of automobile engines, there can occur the problem of damaging the inner walls of the cylinders and causing an engine failure. Thus, it has heretofore been absolutely necessary for the air filter materials of automobile air cleaners to exhibit a favorable dust collection capability. Moreover, when the introduced air is contaminated with carbon soot discharged from diesel engines and the like, sensors for measuring the appropriate flow quantity of air relative to fuel may sometimes malfunction. Thus, the air filter materials of the air cleaners for automobile engines have also been required to exhibit a favorable carbon soot collection capability.
As an air filter material that exhibits a favorable dust collection capability and carbon soot collection capability, an air cleaner filter medium comprising a dense fiber layer and an adjacent fiber layer adjacent thereto has been proposed (see Japanese Patent Application Laid-Open No. Hei 10-337426). Here, the dense fiber layer is a nonwoven fabric made of fine fibers having an average fiber diameter of 3 μm to 15 μm, with an areal density of 20 g/m2 to 120 g/m2. The adjacent fiber layer has an apparent density of 0.1 g/cm3 to 0.2 g/cm3 and an areal density of 10 g/cm2 to 100 g/m2.
A new problem was found, however, that if a large amount of dust is held on an automobile engine air cleaner that uses the air cleaner filter medium described in Japanese Patent Application Laid-Open No. Hei 10-337426 when the engine is driven, the automobile engine air cleaner also changes in the frequency of vibration according to the number of revolutions of the engine and causes such a phenomenon that the dust collected by the air cleaner filter medium penetrates to the air outlet side (dust penetration) around 200 Hz in the frequency of vibration.
For this problem, it seems to be effective to impregnate the air cleaner filter medium with acrylate emulsions or the like, followed by drying so that the fibers constituting the nonwoven fabric are fixed with the resin. If the amount of resin applied increases, however, there can occur the problem that an intended rate of air flow cannot be obtained because the carbon soot collection capability drops and the air cleaner filter medium clogs up with dust.
As above, it has been impossible to maintain the dust collection capability, the dust penetration resistance, and the carbon soot collection capability of a nonwoven-based air filter material, such as the air cleaner filter medium described in Japanese Patent Application Laid-Open No. Hei 10-337426, in a well-balanced manner at high level.
Then, in order to provide an air filter material with a favorable dust collection capability, dust penetration resistance, and carbon soot collection capability in a well-balanced manner, it has been proposed to configure the air filter material as a laminate of three layers of nonwoven fabrics while confining each of the nonwoven fabrics to predetermined ranges in average fiber diameter, areal fiber weight, apparent density, and the amount of resin impregnated (see Japanese Patent Application Laid-Open No. 2004-243250). Specifically, an air filter material formed by laminating a first nonwoven fabric made of fibers having an average fiber diameter of 3 μm to 5 μm, a second nonwoven fabric made of fibers having an average fiber diameter of 9 μm to 15 μm, and a third nonwoven fabric made of fibers having an average fiber diameter of 16 μm to 32 μm has been proposed. Here, the first nonwoven fabric is 35 g/m2 to 55 g/m2 in areal fiber weight, 10 g/m2 to 24 g/m2 in the amount of binder resin applied, and 0.16 g/cm3 to 0.30 g/cm3 in apparent density. The second nonwoven fabric is 20 g/m2 to 100 g/m2 in areal fiber weight, 4 g/m2 to 11 g/m2 in the amount of binder resin applied, and 0.10 g/cm3 to 0.20 g/cm3 in apparent density. Then, the third nonwoven fabric is 55 g/m2 to 100 g/m2 in areal fiber weight, 1 g/m2 to 5 g/m2 in the amount of binder resin applied, and 0.073 g/cm3 to 0.088 g/cm3 in apparent density. Concerning this air filter material, it is mentioned that the first nonwoven fabric is preferably made of splittable staple fibers while spunlace nonwoven fabrics are used as the first and second nonwoven fabrics.
In order for the air filter material of Japanese Patent Application Laid-Open No. 2004-243250 to secure desirable levels of dust penetration resistance, dust collection capability, and carbon soot collection capability, however, the first nonwoven fabric layer must be made of expensive splittable fibers as disclosed in the working Examples thereof. This has resulted in an increase in manufacturing cost. In addition to this, air filter materials have required excellent initial cleaning efficiencies and lives against dust and carbon soot as basic capabilities.